The Card Verification Number is a security feature placed on credit cards and debit cards to ensure that the person entering the card number online or over the phone has possession of the card.
Visa, MasterCard, and Discover list the 3-digit Card Verification Number on the back of the card and to the right of the card number. American Express lists the 4-digit Card Verification Number on the front of the card and to the right of the card number.